U.S. patent number 5,158,063 [Application Number 07/812,705] was granted by the patent office on 1992-10-27 for air-fuel ratio control method for internal combustion engines.
This patent grant is currently assigned to Honda Giken Kogyo K.K.. Invention is credited to Sachito Fujimoto, Fumio Hosoda, Hiroshi Ito, Shunji Takahashi.
United States Patent |
5,158,063 |
Hosoda , et al. |
October 27, 1992 |
Air-fuel ratio control method for internal combustion engines
Abstract
An air-fuel ratio control method for an internal combustion
engine. The air-fuel ratio of an air-fuel mixture supplied to the
engine is feedback-controlled to a predetermined value in response
to an output from the exhaust gas ingredient concentration sensor.
The temperature of at least one component part of the engine, which
should be controlled, is estimated based on the detected
temperature of exhaust gases, engine rotational speed, and engine
load. The air-fuel ratio of the air-fuel mixture is inhibited from
being feedback-controlled but enriched, when the estimated
temperature of any one of the at least one component part of the
engine is higher than a corresponding predetermined value.
Inventors: |
Hosoda; Fumio (Wako,
JP), Fujimoto; Sachito (Wako, JP), Ito;
Hiroshi (Wako, JP), Takahashi; Shunji (Wako,
JP) |
Assignee: |
Honda Giken Kogyo K.K. (Tokyo,
JP)
|
Family
ID: |
18525446 |
Appl.
No.: |
07/812,705 |
Filed: |
December 23, 1991 |
Foreign Application Priority Data
|
|
|
|
|
Dec 28, 1990 [JP] |
|
|
2-417324 |
|
Current U.S.
Class: |
123/676; 123/672;
123/687; 60/277 |
Current CPC
Class: |
F02D
41/0235 (20130101); F02D 41/1446 (20130101); F02D
41/1486 (20130101); F02D 41/1488 (20130101); F02D
2200/0406 (20130101); F02D 2200/0414 (20130101); F02D
2200/0804 (20130101) |
Current International
Class: |
F02D
41/14 (20060101); F02D 41/02 (20060101); F02M
051/00 () |
Field of
Search: |
;123/489,440
;364/431.09,431.05,431.06 ;60/277,274 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Nelli; Raymond A.
Attorney, Agent or Firm: Lessler; Arthur L.
Claims
What is claimed is:
1. An air-fuel ratio control method for an internal combustion
engine including an exhaust passage, an exhaust gas ingredient
concentration sensor arranged in said exhaust passage for detecting
concentration of an exhaust gas ingredient, and at least one
component part temperature of which is to be controlled,
wherein the air-fuel ratio of an air-fuel mixture supplied to said
engine is feedback-controlled to a predetermined value in response
to an output from said exhaust gas ingredient concentration sensor,
and when it is determined that said engine is in a predetermined
high load operating condition and at the same time said at least
one component part of said engine is in a predetermined high
temperature state, the air-fuel ratio of said air-fuel mixture is
inhibited from being feedback-controlled but enriched instead,
the improvement comprising the steps of:
(1) detecting a temperature of exhaust gases emitted from said
engine;
(2) detecting rotational speed of said engine;
(3) detecting load on said engine;
(4) estimating temperature of said at least one component part of
said engine based on said temperature of exhaust gases, engine
rotational speed, and engine load, detected at the above steps (1)
to (3); and
(5) determining that said at least one component part of said
engine is in said predetermined high temperature state when said
estimated temperature of any one of said at least one component
part of said engine is higher than a corresponding predetermined
value.
2. An air-fuel ratio control method according to claim 1, wherein
said estimated temperature T of said at least one component part of
said engine is calculated by the use of the following equation:
where TE represents said detected temperature of exhaust gases, KNE
represents an engine rotational speed-dependent correction
coefficient set according to said detected engine rotational speed,
and KPB represents an engine load-dependent correction coefficient
set according to said detected engine load.
3. An air-fuel ratio control method according to claim 2, wherein
said engine rotational speed-dependent correction coefficient KNE
is set to a larger value as said engine rotational speed is higher,
and said engine load-dependent correction coefficient KPB is set to
a larger value as said engine load is higher.
4. An air-fuel ratio control method according to claim 1, 2, or 3,
wherein said estimated temperature of said at least one component
part of said engine is obtained by averaging a plurality of values
of said estimated temperature, and the speed of said averaging is
changed depending on said engine load.
5. An air-fuel ratio control method according to claim 4, wherein
said engine load is intake pipe absolute pressure corrected
according to intake air temperature.
6. An air-fuel ratio control method according to claim 1, wherein
the air-fuel ratio of an air-fuel mixture supplied to said engine
is controlled to a first value richer than a stoichiometric
air-fuel ratio after it was determined that said at least one
component part of said engine is in said predetermined high
temperature state and before a predetermined time period elapses
thereafter, and the air-fuel ratio of said air-fuel mixture is
controlled to a second value richer than said first value after
said predetermined time period elapses.
7. An air-fuel ratio control method according to claim 6, wherein
said enriching of the air-fuel ratio of said air-fuel mixture is
carried out by multiplying a basic amount of fuel supplied to said
engine, which is determined according to said engine rotational
speed detected and intake pipe pressure, by a predetermined
enriching coefficient, said predetermined enriching coefficient
being determined based on said engine rotational speed detected and
said intake pipe pressure.
8. An air-fuel ratio control method according to claim 7, wherein
said predetermined enriching coefficient is set to a value read
from a map set according to said engine rotational speed detected
and said intake pipe pressure before said predetermined time period
elapses to thereby control the air-fuel ratio of said air-fuel
mixture to said first value, said predetermined enriching
coefficient being set to a value obtained by multiplying said value
read from said map by an enriching coefficient after said
predetermined time period has elapsed to thereby control the
air-fuel ratio of said air-fuel mixture to said second value.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an air-fuel ratio control method for
internal combustion engines, and more particularly to a method of
controlling the air-fuel ratio of an air-fuel mixture supplied to
an internal combustion engine when the engine is in a high load
operating condition.
2. Prior Art
It is conventionally known to control the air-fuel ratio of an
air-fuel mixture supplied to an internal combustion engine
(hereinafter referred to as "the supply air-fuel ratio") to a
stoichiometric air-fuel ratio or its vicinity when load on the
engine is relatively low, and enrich the supply air-fuel ratio to
prevent the temperature of the engine from rising to an excessive
degree by utilizing the effects of cooling by fuel in the air-fuel
mixture supplied to the engine, when the load on the engine is
high. To carry out this air-fuel ratio control method, the
following techniques have conventionally been proposed:
(1) A desired exhaust gas temperature is set based on the amount of
intake air, engine rotational speed, and engine coolant
temperature, and the supply air-fuel ratio is controlled such that
the actual exhaust gas temperature becomes equal to the set desired
exhaust gas temperature (Japanese Provisional Patent Publication
(Kokai) No. 60-90940).
(2) An exhaust gas temperature is estimated based on the amount of
intake air or engine rotational speed and the supply air-fuel ratio
is enriched to a greater extent as the estimated exhaust gas
temperature is higher (Japanese Patent Publication (Kokoku) No.
62-54977).
(3) The temperature of a catalytic converter provided in an
internal combustion engine is estimated based on the amount of
intake air and the supply air-fuel ratio, whereby the supply
air-fuel ratio is controlled so as to prevent an excessive rise in
the temperature of the catalytic converter (Japanese Provisional
Patent Publication (Kokai) No. 62-203965).
(4) An engine temperature is estimated based on engine rotational
speed, load on the engine, and the supply air-fuel ratio, and
enriching of the supply air-fuel ratio is controlled depending on
the estimated engine temperature (Japanese Provisional Patent
Publication (Kokai) No. 3-18643).
Further, to determine whether an oxygen concentration sensor
arranged in an exhaust passage of the engine is activated or not,
the following technique has also been proposed:
(5) The temperature of the oxygen concentration sensor is estimated
based on the amount of intake air and outside air temperature
(Japanese Provisional Patent Publication (Kokai) No. 1-219340).
According to the above techniques (2) to (5). the exhaust gas
temperature or the temperature of an engine component part is
estimated based on engine operating parameters, such as the amount
of intake air and engine rotational speed, but the actual exhaust
gas temperature is not detected. Therefore, there is a possibility
of an estimated value of the exhaust gas temperature becoming
largely different from an actual value of same. To overcome this
disadvantage, it is required to make wider the engine operating
region in which the supply air-fuel ratio should be enriched
(hereinafter referred to as "the high load enriching region"), i.e.
set a reference temperature for determining whether the supply
air-fuel ratio should be enriched to a lower value. As a result,
there can be cases where the air-fuel ratio is unnecessarily
enriched, which results in degradation of fuel consumption and
exhaust emission characteristics.
Further, according to the above technique (1), the temperature of
the exhaust system is determined only by detecting the exhaust gas
temperature, and the desired exhaust gas temperature is set based
on the amount of intake air, engine rotational speed, intake air
temperature, and engine coolant temperature to control the supply
air-fuel ratio such that the detected exhaust gas temperature
becomes equal to the desired exhaust gas temperature. However, the
temperature of engine component parts, which may rise to an
excessive degree, varies not only by the exhaust gas temperature
but also by the volume of hot exhaust gases. More specifically,
even if the exhaust gas temperature remains unchanged, the rate of
rise in the temperature of engine component parts tends to be lower
when the volume of exhaust gases (which may be determined by the
engine rotational speed and engine load) is smaller than when the
volume of exhaust gases is larger. In the case of a three-way
catalyst, for example, it has been found that even if the exhaust
gas temperature remains unchanged, when the engine is in a high
load and high engine rotational speed condition, in which the
volume of exhaust gases is larger, the rate of thermal conduction
to the three-way catalyst tends to increase due to the flow of an
increased volume of hot exhaust gases to cause the temperature of
the three-way catalyst to rise at a higher rate. On the other hand,
when the volume of exhaust gases is smaller, the temperature of the
three-way catalyst rises at a lower rate in spite of the presence
of the flow of hot exhaust gases. Further, it has also been found
that due to difference in thermal capacity between engine component
parts, the amount of thermal conduction therethrough varies with
the engine component parts, which causes the temperatures of the
engine component parts to rise at different rates as time elapses.
Therefore, there is room for improvement of this technique (1)
concerning fuel consumption and exhaust emission
characteristics.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an air-fuel ratio
control method for an internal combustion engine which is capable
of improving the accuracy of estimated temperatures of component
parts of the engine, and reducing fuel consumption and emission of
CO when the engine is in a high load operating condition.
To attain the above object, the invention provides an air-fuel
ratio control method for an internal combustion engine including an
exhaust passage, an exhaust gas ingredient concentration sensor
arranged in the exhaust passage for detecting concentration of an
exhaust gas ingredient, and at least one component part temperature
of which is to be controlled,
wherein the air-fuel ratio of an air-fuel mixture supplied to the
engine is feedback-controlled to a predetermined value in response
to an output from the exhaust gas ingredient concentration sensor,
and when it is determined that the engine is in a predetermined
high load operating condition and at the same time the at least one
component part of the engine is in a predetermined high temperature
state, the air-fuel ratio of the air-fuel mixture is inhibited from
being feedback-controlled but enriched instead.
The air-fuel ratio control method according to the invention is
characterized by comprising the steps of:
(1) detecting a temperature of exhaust gases emitted from the
engine;
(2) detecting rotational speed of the engine;
(3) detecting load on the engine;
(4) estimating temperature of the at least one component part of
the engine based on the temperature of exhaust gases, engine
rotational speed, and engine load, detected at the above steps (1)
to (3); and
(5) determining that the at least one component part of the engine
is in the predetermined high temperature state when the estimated
temperature of any one of the at least one component part of the
engine is higher than a corresponding predetermined value.
Preferably, the estimated temperature T of the at least one
component part of the engine is calculated by the use of the
following equation:
where TE represents the detected temperature of exhaust gases, KNE
represents an engine rotational speed-dependent correction
coefficient set according to the detected engine rotational speed,
and KPB represents an engine load-dependent correction coefficient
set according to the detected engine load.
More preferably, the engine rotational speed-dependent correction
coefficient KNE is set to a larger value as the engine rotational
speed is higher, and the engine load-dependent correction
coefficient KPB is set to a larger value as the engine load is
higher.
Preferably, the estimated temperature of the at least one component
part of the engine is obtained by averaging a plurality of values
of the estimated temperature, and the speed of the averaging is
changed depending on the engine load.
More preferably, the engine load is intake pipe absolute pressure
corrected according to intake air temperature.
Preferably, the air-fuel ratio of an air-fuel mixture supplied to
the engine is controlled to a first value richer than a
stoichiometric air-fuel ratio after it was determined that the at
least one component part of the engine is in the predetermined high
temperature state and before a predetermined time period elapses
thereafter, and the air-fuel ratio of the air-fuel mixture is
controlled to a second value richer than the first value after the
predetermined time period elapses.
More preferably, the enriching of the air-fuel ratio of the
air-fuel mixture is carried out by multiplying a basic amount of
fuel supplied to the engine, which is determined according to the
engine rotational speed detected and intake pipe pressure, by a
predetermined enriching coefficient, the predetermined enriching
coefficient being determined based on the engine rotational speed
detected and the intake pipe pressure.
Further preferably, the predetermined enriching coefficient is set
to a value read from a map set according to the engine rotational
speed detected and the intake pipe pressure before the
predetermined time period elapses to thereby control the air-fuel
ratio of the air-fuel mixture to the first value, the predetermined
enriching coefficient being set to a value obtained by multiplying
the value read from the map by an enriching coefficient after the
predetermined time period has elapsed to thereby control the
air-fuel ratio of the air-fuel mixture to the second value.
The above and other objects, features, and advantages of the
invention will be more apparent from the ensuing detailed
description taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of the whole arrangement of a fuel
supply control system for an internal combustion engine to which is
applied the control method according to the invention;
FIGS. 2a and 2b are flowcharts of a program for performing
calculation of estimated temperatures of component parts of the
engine;
FIG. 3 is a flowchart of a subroutine for correcting the intake
pipe absolute pressure depending on intake air temperature;
FIG. 4 is a diagram showing a table for calculating an intake air
temperature-dependent correction coefficient (KTEXG);
FIG. 5 is a flowchart of a subroutine for calculating an estimated
temperature value (TCAT) of a three-way catalyst;
FIG. 6a is a diagram showing a table for calculating a coefficient
(KNCAT) for correcting an exhaust gas temperature value;
FIG. 6b is a diagram showing a table for calculating a coefficient
(KPBCAT) for correcting an exhaust gas temperature value;
FIG. 6c is a diagram showing a table for calculating an averaging
coefficient (TREFO);
FIG. 7 is a flowchart of a subroutine for calculating an estimated
temperature value (TEXM) of an exhaust pipe;
FIG. 8a is a diagram showing a table for calculating a coefficient
(KNEXM) for correcting an exhaust gas temperature value;
FIG. 8b is a diagram showing a table for calculating a coefficient
(KPBEXM) for correcting an exhaust gas temperature value;
FIG. 8c is a diagram showing a table for calculating a coefficient
(KVEXM) for correcting an exhaust gas temperature value;
FIG. 9 is a flowchart of a subroutine for calculating an estimated
temperature value (TPIS) of pistons;
FIG. 1Oa is a diagram showing a table for calculating a coefficient
(KNPIS) for correcting an exhaust gas temperature value;
FIG. 1Ob is a diagram showing a table for calculating a coefficient
(KPBPIS) for correcting an exhaust gas temperature value;
FIG. 1Oc is a diagram showing a table for calculating a variable
(DTPIS) for correcting an exhaust gas temperature value;
FIG. 11 is a flowchart of a subroutine for calculating an estimated
temperature value (TEXV) of exhaust valves;
FIG. 12a is a diagram showing a table for calculating a coefficient
(KNEXV) for correcting an exhaust gas temperature value;
FIG. 12b is a diagram showing a table for calculating a coefficient
(KPBEXV) for correcting an exhaust gas temperature value;
FIG. 12c is a diagram showing a table for calculating a variable
(DTEXV) for correcting an exhaust gas temperature value;
FIG. 13 is a flowchart of a subroutine for setting a high
temperature flag (FXAVE);
FIGS. 14a and 14b are a flowchart of a program for calculating a
high load enriching coefficient (KWOT);
FIG. 15 is a diagram showing a table for calculating reference
values (PBWOT1, PBWOT2) for determining whether the engine is in a
high load operating condition;
FIG. 16 is a diagram showing a table for calculating an enriching
coefficient (XWOTR) and a leaning coefficient (XWOTL); and
FIG. 17 is a diagram showing a table for calculating an engine
coolant temperature-dependent enriching coefficient (XWOTTW).
DETAILED DESCRIPTION
The method according to the invention will now be described in
detail with reference to the drawings showing an embodiment
thereof.
Referring first to FIG. 1, there is shown the whole arrangement of
a fuel supply control system which is adapted to carry out the
control method of this invention. In the figure, reference numeral
1 designates an internal combustion engine. In an intake pipe 2 of
the engine 1, there is arranged a throttle body 3 accommodating a
throttle valve 3' therein. A throttle valve opening (.theta.TH)
sensor 4 is connected to the throttle valve 3' for generating an
electric signal indicative of the sensed throttle valve opening and
supplying same to an electronic control unit (hereinafter referred
to as "the ECU") 5.
Fuel injection valves 6 ar each provided for each cylinder, not
shown, and arranged in the intake pipe 2 between the engine 1 and
the throttle valve 3, and at a location slightly upstream of an
intake valve, not shown. The fuel injection valves 6 are connected
to a fuel pump, not shown, and electrically connected to the ECU 5
to have their valve opening periods controlled by signals
therefrom.
Further, an intake pipe absolute pressure (PBA) sensor 8 is
provided in communication with the interior of the intake pipe 2
via a conduit 7 at a location immediately downstream of the
throttle valve 3' for supplying an electric signal indicative of
the sensed absolute pressure to the ECU 5. An intake temperature
(TA) sensor 9 is inserted into the intake pipe 2 at a location
downstream of the intake pipe absolute pressure sensor 8 for
supplying an electric signal indicative of the sensed intake
temperature TA to the ECU 5.
An engine coolant temperature (TW) sensor 10, which may be formed
of a thermistor or the like, is mounted in the cylinder block of
the engine 1 for supplying an electric signal indicative of the
sensed engine coolant temperature TW to the ECU 5. An engine
rotational speed (NE) sensor 11 and a cylinder-discriminating (CYL)
sensor 12 are arranged in facing relation to a camshaft or a
crankshaft of the engine 1, neither of which is shown. The engine
rotational speed sensor 11 generates a pulse as a TDC signal pulse
at each of predetermined crank angles whenever the crankshaft
rotates through 180 degrees, while the cylinder-discriminating
sensor 12 generates a pulse at a predetermined crank angle of a
particular cylinder of the engine, both of the pulses being
supplied to the ECU 5.
A three-way catalyst 14 is arranged within an exhaust pipe 13
connected to the cylinder block of the engine 1 for purifying
noxious components such as HC, CO and NO.sub.X. An O.sub.2 sensor
15 as an exhaust gas ingredient concentration sensor is mounted in
the exhaust pipe 13 at a location upstream of the three-way
catalyst 14, for detecting the oxygen concentration in the exhaust
gases and supplying an electric signal indicative of a detected
value of the oxygen concentration to the ECU 5. Further, an exhaust
gas temperature sensor 16 is mounted in the exhaust pipe 13 at a
location upstream of the O.sub.2 sensor 15 for supplying a signal
indicative of a detected value of the exhaust gas temperature to
the ECU 5. A vehicle speed (VSP) sensor 17, which is also connected
to the ECU 5, detects a travelling speed (vehicle speed VSP) of an
automotive vehicle on which the engine is installed, and supplies a
signal indicative of the vehicle speed VSP to the ECU 5.
The ECU 5 comprises an input circuit 5a having the functions of
shaping the waveforms of input signals from various sensors,
shifting the voltage levels of sensor output signals to a
predetermined level, converting analog signals from analog-output
sensors to digital signals, and so forth, a central processing unit
(hereinafter referred to as "the CPU") 5b, memory means 5c storing
various operational programs which are executed in the CPU 5b and
for storing results of calculations therefrom, etc., and an output
circuit 5d which outputs driving signals to the fuel injection
valves 6.
The CPU 5b operates in response to the above-mentioned signals from
the sensors to determine operating conditions in which the engine 1
is operating such as an air-fuel ratio feedback control region and
open-loop control regions, and calculates, based upon the
determined operating conditions, the valve opening period or fuel
injection period T.sub.OUT over which the fuel injection valves 6
are to be opened by the use of the following equation (1) in
synchronism with inputting of TDC signal pulses to the ECU 5:
where Ti represents a basic fuel amount, more specifically a basic
fuel injection period, and is read from a Ti map set according to
the engine rotational speed NE and the intake pipe absolute
pressure PBA and stored in the memory means 5c. KWOT is a high load
enriching coefficient for enriching an air-fuel mixture supplied to
the engine when the throttle valve 3' is substantially fully open,
and determined in such a manner as described hereinbelow with
reference to FIGS. 14a and 14b. KTW is an engine coolant
temperature-dependent fuel increasing coefficient for enriching the
air-fuel mixture when the engine coolant temperature TW is equal to
or lower than a predetermined value. K.sub.O2 is an air-fuel ratio
feedback correction coefficient which is set responsive to the
oxygen concentration in the exhaust gases when the engine is in a
feedback control region, and set, when the engine is not in the
feedback control region but in any of particular regions (open loop
control regions), to a value peculiar thereto. A state in which the
engine is in a predetermined high load operating condition and the
temperature of any of predetermined component parts of the engine
is high is included in the open loop control regions.
K.sub.1 and K.sub.2 are other correction coefficients and
correction variables, respectively, which are calculated based on
various engine parameter signals to such values as to optimize
characteristics of the engine such as fuel consumption and
accelerability depending on engine operating conditions.
The CPU 5b supplies the fuel injection valves 6 with driving
signals based on the fuel injection period T.sub.OUT calculated as
above via the output circuit 5d.
FIG 2 shows a program for calculating estimated temperature values
of component parts of the engine, i.e. the three-way catalyst 14,
the exhaust pipe 13, pistons in the cylinders, and the exhaust
valves, and setting, based on the estimated temperature values,
first and second enriching flags FHSFE1, FHSFE2 for enriching the
supply air-fuel ratio. This program is executed at constant time
intervals (e.g. of 80 millisec.).
At a step S1, an output from the exhaust gas temperature sensor 16
is read. The exhaust gas temperature sensor 16 used in the present
embodiment is comprised of a thermistor. The output from the
exhaust gas temperature sensor 16 is in a non-linear relationship
to the exhaust gas temperature. Therefore, the sensor output is
converted into the exhaust gas temperature (TE) by the use of a
table stored in the memory means 5c. In performing the conversion,
linear interpolation is effected for values of the sensor output
other than the values of same stored beforehand.
At a step S2, the intake pipe absolute pressure PBA is corrected
based on the intake air temperature TA, by a subroutine shown in
FIG. 3. More specifically, an intake air temperature-dependent
correction coefficient KTAEXG is calculated according to the intake
air temperature TA, and the intake pipe absolute pressure PBA is
multiplied by the coefficient KTAEXG to obtain corrected intake
pipe absolute pressure PBAEX. The correction coefficient KTAEXG is
read from a KTAEXG table in FIG. 4, in which coefficient values
KTAEXG0 to KTAEXG2 (e.g. 1.15, 1.0, 0.95) are set corresponding to
predetermined values TAEXG0 to TAEXG2 (e.g. -10.degree. C.,
30.degree. C., 50.degree. C.) of the intake air temperature TA. For
values other than the predetermined values TAEXG0 to TAEXG2 of the
intake air temperature TA, interpolation is carried out. Reading of
values from other tables, referred to hereinafter, is similarly
carried out. The intake air temperature-dependent correction of the
intake pipe absolute pressure PBA is performed in order to take
into consideration variation in charging efficiency caused by the
intake air temperature TA. More specifically, as the charging
efficiency is lower, the weight of intake air which is used in
burning of the air-fuel mixture decreases, so that combustion
temperature tends to become lower accordingly. In view of this
tendency, the intake pipe absolute pressure PBA is corrected such
that its value decreases to such an extend as corresponds to an
amount of decrease in the weight of the intake air, whereby the
accuracy of calculation of estimated values of increased
temperatures of component parts in the exhaust system of the engine
is improved.
Referring again to FIG. 2, at a step S3, an estimated temperature
value (hereinafter simply referred to as the "catalyst
temperature") TCAT of the three-way catalyst 14 is calculated by a
subroutine shown in FIG. 5.
In FIG. 5, at a step S31, it is determined whether or not the
engine is in a starting mode. If the answer to this question is
affirmative (YES), i.e. if the engine is in the starting mode, both
the catalyst temperature TCAT and an average value TCATave thereof
are set to a predetermined initial value TCATO (e.g. 400.degree.
C.) at steps S32 and S33, and then the program proceeds to a step
S37.
If the answer to the question of the step S31 is negative (NO),
i.e. if the engine is not in the starting mode, correction
coefficients KNCAT and KPBCAT for converting the exhaust gas
temperature TE detected into the catalyst temperature TCAT are
calculated at steps S34 and S35, and then the catalyst temperature
TCAT is obtained by multiplying the exhaust gas temperature TE by
these correction coefficients at a step S36.
KNCAT is an engine rotational speed-dependent correction
coefficient set according to the engine rotational speed. As shown
in FIG. 6a, a value thereof is read from a KNCAT table in which
values KNCAT0 to KNCAT2 are set corresponding to predetermined
values NCAT0 and NCAT2 of the engine rotational speed. The
correction coefficient KNCAT assumes a larger value as the engine
rotational speed NE is higher, and if NE=NCAT1 (e.g. 3,000 rpm),
KNCAT=1.0.
KPBCAT is an engine load-dependent correction coefficient which is
set according to the corrected intake pipe absolute pressure PBAEX.
As shown in FIG. 6b, a value thereof is read from a KPBCAT table in
which values KPBCAT0 to KPBCAT2 are set corresponding to
predetermined values PBACAT0 to PBACAT2 of the corrected intake
pipe absolute pressure PBAEX. The correction coefficient KPBCAT
assumes a larger value as the corrected intake pipe absolute
pressure PBAEX is higher, and if PBAEX=PBACAT1 (e.g. 510 mmHg),
KPBCAT=1.0.
The reason for setting the correction coefficients KNCAT and KPBCAT
such that they increase as the engine rotational speed NE and the
intake pipe absolute pressure PBA increase is that the rate of
thermal conduction varies with the volume of exhaust gases, which
causes a resulting variation in the temperature of a component part
(in this case the three-way catalyst) of the engine. In estimating
temperatures of the exhaust pipe, the pistons in the cylinders, and
the exhaust valves, which will be described later, the exhaust gas
temperature TE will be converted into the respective temperatures
in a similar manner, i.e. by correcting the exhaust gas temperature
TE according to the volume of exhaust gases which is determined by
the engine rotational speed NE and the engine load.
At a step S37, an average value TCATave of the catalyst temperature
is calculated by the use of the following equation (2): ##EQU1##
wherein (n) and (n-1) indicate that the values are obtained in the
present loop and the last loop, respectively. TREFO is an averaging
coefficient which determines the rate of contribution of a present
value TCAT(n) of the catalyst temperature to a present value
TCATave(n) of the average value. As TREFO increases, TCAT(n)
contributes to TCATave(n) at a larger rate, so that the averaging
speed increases. In this connection, in the present embodiment, the
averaging coefficient TREFO is read from a TREFO table, as shown in
FIG. 6c, in which values thereof are set according to the corrected
intake pipe absolute pressure PBAEX.
In the TREFO table, predetermined values TREFOL and TREFOH (e.g.
87, 1190) are set corresponding to predetermined values PBTREFL and
PBTREFH (e.g. 150 mmHg, 480 mmHg) of the corrected intake pipe
absolute pressure. The averaging value TREFO increases as the
engine load is higher. This takes into consideration the fact that
as the engine load is lower, the volume of exhaust gases (the
amount of exhaust gases emitted per unit time period) is smaller,
which results in a smaller rate of change in the temperature of a
component part. By thus setting the averaging coefficient TREFO, it
is possible to obtain a proper average value TCATave corresponding
to the engine load.
Further, an amount of rise in the temperature of a component part
of the engine per unit time period is dependent not only on the
engine rotational speed and the engine load but also on the thermal
capacity of the component part, each component part of the engine
having a thermal capacity peculiar thereto. Therefore, the
averaging coefficient TREF is set for each component part,
separately. In estimating the temperatures of the exhaust pipe, the
pistons in the cylinders, and the exhaust valves, which will be
described later, the averaging coefficient TREF is set for each of
the component parts in a similar manner according to an amount of
rise in the temperature of the component part of the engine per
unit time period.
Referring again to FIG. 2, at a step S4, an estimated temperature
value (herinafter simply referred to as the "exhaust pipe
temperature") TEXM of the exhaust pipe is calculated by a
subroutine shown in FIG. 7.
Similarly to the program of FIG. 5, if the engine is in the
starting mode (the answer to the question of the step S41 is
affirmative (YES)), both the exhaust pipe temperature TEXM and an
average value TEXMave thereof are set to a predetermined initial
value TEXMO (e.g. 400.degree. C.) at steps S42 and S43. On the
other hand, if the engine is not in the starting mode (the answer
to the question of the step S41 is negative (NO)), correction
coefficients KNEXM, KPBEXM, and KVEXM for converting the exhaust
gas temperature TE detected into the exhaust pipe temperature TEXM
are calculated at steps S44 to S46, and then the exhaust pipe
temperature TEXM is obtained by multiplying the exhaust gas
temperature RE by these coefficients at a step S47.
KNEXM and KPBEXM are engine rotational speed-dependent and engine
load-dependent correction coefficients for obtaining the exhaust
pipe temperature which correspond to the engine rotational
speed-dependent correction coefficient KNCAT and the load-dependent
correction coefficient KPBCAT. Values thereof are read from a KNEXM
table shown in FIG. 8a and a KPBEXM table shown in FIG. 8b. In the
KNEXM table, similarly to the aforementioned KNCAT table, values
KNEXM0 to KNEXM2 are set corresponding to predetermined values
NEXM0 to NEXM2 of the engine rotational speed, and if NE=NEXM1
(e.g. 3,500 rpm), KNEXM1=1.0. In the KPBEXM table, similarly to the
KPBCAT table, values KPBEXM0 to KPBEXM2 are set corresponding to
predetermined values PBAEXM0 to PBAEXM2 of the corrected exhaust
pipe absolute pressure PBAEX, and if PBAEX=PBAEXM1 (e.g. 510 mmHg),
KPBEXM1=1.0.
KVEXM is a vehicle speed-dependent correction coefficient which is
set according to the vehicle speed VSP. As shown in FIG. 8c, values
KVEXM0 to KVEXM2 are set corresponding to predetermined values
VEXM0 to VEXM2 of the vehicle speed. The coefficient KVEXM
decreases as the vehicle speed VSP is lower, and if VSP=VEXM1 (e.g.
120 km/h), KVEXM=1.0. This is to lower the estimated temperature
value of the exhaust pipe when the vehicle speed VSP is higher,
since the exhaust pipe of the engine is more cooled as the vehicle
travels at a higher speed.
At a step S48, an average value TEXave of the exhaust pipe
temperature is calculated by the use of the following equation (b
3): ##EQU2##
The equation (3) is similar to the equation (2), and where the
averaging coefficient TREF1 is set to a fixed value, e.g. 20.
Referring again to FIG. 2, at a step S5, an estimated temperature
value (hereinafter simply referred to as the "piston temperature")
TPIS of the pistons in the cylinders is calculated by a subroutine
shown in FIG. 9.
In the program of FIG. 9, the piston temperature TPIS and an
average value TPISave thereof are calculated in a manner similar to
those of the programs described with reference to FIGS. 5 and 7.
More specifically, if the engine is in the starting mode (the
answer to the question of a step S51 is affirmative (YES)), both
the piston temperature TPIS and the average value TPISave thereof
are set to their predetermined initial value TPISO (e.g. 80.degree.
C.) at steps S52 and S53. On the other hand, if the engine is not
in the starting mode (the answer to the question of the step S51 is
negative (NO)), correction coefficients KNPIS and KPBPIS and a
correction variable DTPIS for converting the exhaust gas
temperature TE detected into the piston temperature TPIS are
calculated at steps S54 to S56, and the piston temperature TPIS is
obtained at a step S57 by applying these correction coefficient and
variable to the following equation (4): ##EQU3## where KPIS is a
converting coefficient which is set e.g. to approx. 0.125, and CPIS
is a converting variable which is set e.g. to approx. 35.degree.
C.
KNPIS and KPBPIS are engine rotational speed-dependent and engine
load-dependent correction coefficients for obtaining the piston
temperature, respectively. Values thereof are read from a KNPIS
table shown in FIG. 10a and a KPBPIS table shown in FIG. 10b. In
the KNIPS table, similarly to the KNCAT table, values KNPIS0 to
KNPIS2 are set corresponding to predetermined values NPIS0 to NPIS
2 of the engine rotational speed, and if NE=NPIS1 (e.g. 3,500 rpm),
KNPIS1=1.0. In the KPBPIS table, similarly to the KPBCAT table,
values KPBPIS0 to KPBPIS2 are set corresponding to predetermined
values PBAPIS0 to PBAPIS2 of the corrected intake pipe absolute
pressure, and if PBAEX=PBAPIS1 (e.g. 510 mmHg), KPBPIS=1.0.
DTPIS is a correction variable set according to the engine coolant
temperature TW, and a value thereof is read from a DTPIS table
shown in FIG. 10c in which values DTPIS0 and DTPIS1 (e.g.
30.degree. C. and 115.degree. C., respectively) are set
corresponding to predetermined values TWPIS0 and TWPIS1 (e.g.
50.degree. C. and 120.degree. C., respectively) of the engine
coolant temperature.
At a step S58, an average value TPISave of the piston temperature
is calculated by the use of the following equation (5), followed by
terminating the present subroutine: ##EQU4##
The equation (5) is similar to the equation (3), and where the
averaging coefficient TREF2 is set to a fixed value, e.g. approx.
8.
Referring again to FIG. 2, at a step S6, an estimated temperature
(hereinafter simply referred to as the "exhaust valve temperature")
TEXV of the exhaust valves is calculated by a subroutine shown in
FIG. 11.
In the program of FIG. 11, the exhaust valve temperature TEXV and
an average value TEXVave thereof are calculated in a manner similar
to that of the program described with reference to FIG. 9. More
specifically, if the engine is in the starting mode (the answer to
the question of a step S61 is affirmative (YES)), both the exhaust
valve temperature TEXV and the average value TEXVave thereof are
set to a predetermined initial value TEXVO (e.g. 200.degree. C.) at
steps S62 and S63. On the other hand, if the engine is not in the
starting mode (the answer to the question of the step S61 is
negative (NO)), correction coefficients KNEXV and KPBEXV and a
correction variable DTEXV for converting the exhaust gas
temperature TE detected into the exhaust valve temperature TEXV are
calculated at steps S64 to S66, and the exhaust valve temperature
TEXV is obtained at a step S67 by applying these correction
coefficients and variable to the following equation (6):
##EQU5##
where KEXV is a converting coefficient which is set e.g. to approx.
0.185, and CEXV is a converting variable which is set e.g. to
approx. 80.degree. C.
KNEXV and KPBEXV are engine rotational speed-dependent and engine
load-dependent correction coefficients for obtaining the exhaust
valve temperature, respectively. Values thereof are read from a
KNEXV table shown in FIG. 12a and a KPBEXV table shown in FIG. 12b.
In the KNEXV table, similarly to the KNCAT table, values KNEXV0 to
KNEXV2 are set corresponding to predetermined values NEXV0 to NEXV2
of the engine rotational speed, and if NE=NEXV1 (e.g. 3,500 rpm),
KNEXV1=1.0. In the KPBEXV table, similarly to the KPBEXV table,
values KPBEXV0 to KPBEXV2 are set corresponding to predetermined
values PBAEXV0 to PBAEXV2 of the corrected intake pipe absolute
pressure, and if PBAEX=PBAEXV1 (e.g. 510 mmHg), KPBEXV=1.0.
DTEXV is a correction variable set according to the engine coolant
temperature TW, and a value thereof is read from a DTEXV table
shown in FIG. 12c in which values DTEXV0 and DTEXV1 (e.g.
10.degree. C. and 140.degree. C., respectively) are set
corresponding to predetermined values TWEXV0 and TWEXV1 (e.g.
85.degree. C. and 110.degree. C., respectively) of the engine
coolant temperature.
At a step S68, an average value TEXVave of the exhaust valve
temperature is calculated by the use of the following equation (7),
followed by terminating the present subroutine: ##EQU6##
The equation (7) is similar to the equation (3), and where the
averaging coefficient TREF3 is set to a fixed value, e.g. approx.
20.
According to the steps S3 to S6 described above, the exhaust gas
temperature TE detected is corrected by the engine rotational
speed-dependent correction coefficients (KNCAT, KNEXM, KNPIS,
KNEXV), the engine load-dependent correction coefficients (KPBACAT,
KPBEXM, KPBPIS, KPBEXM), etc to thereby obtain the estimated
temperature values (TCAT, TEXM, TPIS, TEXM) of the component parts
(three-way catalyst, exhaust pipe, pistons, exhaust valves) of the
engine. This enables to accurately estimate the temperatures of the
component parts which reflect the influence of the volume of
exhaust gases.
Further, it is determined, based on these estimated temperature
values, whether or not the supply air-fuel ratio should be
enriched, as will be described later, which enables to prevent
unnecessary enriching of the supply air-fuel ratio, and reduce fuel
consumption and emission of CO.
Referring again to FIG. 2, at a step S7, a high temperature flag
FXAVE for indicating that the supply air-fuel ratio should be
enriched is set according to a subroutine shown in FIG. 13.
At steps S71 to S74 in FIG. 13, it is determined whether or not the
average value TCATave of the catalyst temperature calculated as
above is higher than a predetermined value TCATG (e.g. 920.degree.
C.), whether or not the average value TEXMave of the exhaust pipe
temperature is higher than a predetermined value TEXMG (e.g.
950.degree. C.), whether or not the average value TPISave of the
piston temperature is higher than a predetermined value TPISG (e.g.
300.degree. C.), and whether or not the average value TEXVave of
the exhaust valve temperature is higher than a predetermined value
TEXVG (e.g. 350.degree. C.), respectively. If any of the answers to
the questions of the steps S71 to S74 is affirmative (YES), the
high temperature flag FXAVE indicative of a high temperature state
of the component part of the engine is set to a value of 1 at a
step S76, whereas if all the answers are negative (NO), the flag
FXAVE is set to a value of 0 at a step S75, followed by terminating
the present program.
Referring again to FIG. 2, at a step S8, it is determined whether
or not the high temperature flag FXAVE is equal to 1. If the answer
to this question is negative (NO), i.e. if FXAVE=0, a counter CHSFE
for measuring a time period elapsed after the high temperature flag
FXAVE was changed from 0 to 1 is set a predetermined value CHSFE0
(e.g. 250). Then, it is determined at a step S1O whether or not a
second high load flag FWOT2, which is set to a value of 1 when the
engine is in a high load operating condition in which the intake
pipe absolute pressure PBA assumes a value higher than a second
reference value PBWOT2, is equal to 1. The second reference value
PBWOT2 is set, as shown by the broken line in FIG. 15, according to
the engine rotational speed NE. In the figure, PBWOT1 is a first
reference value which is also set according to the engine
rotational speed NE. In this connection, when the engine rotational
speed NE is lower than a predetermined value NHSFE shown therein,
PBWOT2= PBWOT1. If the intake pipe absolute pressure PBA is higher
than the first reference value, a first high load flag FWOT1 is set
to a value of 1. The first and second high load flags FWOT1 and
FWOT2 are used in a program shown in FIGS. 14a and 14b.
Referring again to FIG. 2, if the answer to the step S1O is
negative (NO), i.e. if FWOT2=0, which means that the engine is not
in the high load operating condition, a timer tMWOTX is set to a
predetermined time period TMWOTX0 (e.g. 90 seconds) and started at
a step S11, and second and first enriching flags FHSFE2, FHSFE1 are
set to a value of 0 at steps S12, S13, respectively, followed by
terminating the present program.
If the answer to the question of the step S1O is affirmative (YES),
i.e. if FWOT2=1, which means that the engine is in the high load
operating condition, it is determined at a step S14 whether or not
the count value of the timer tMWOTX is equal to 0. If the answer to
this question is negative (NO), i.e. if the predetermined time
period TMWOTX0 has not elapsed after the second high load flag
FWOT2 was changed from 0 to 1, the present program is immediately
terminated. On the other hand, if the answer to the question of the
step S14 is affirmative (YES), i.e. the predetermined time period
TMWOTX0 has elapsed, it is determined at a step S20 whether or not
the first enriching flag FHSFE1 is equal to 1. If the answer to
this question is affirmative (YES), the present program is
immediately terminated, whereas if the answer is negative (NO), the
first enriching flag FHSFE1 is set to a value of 1 at a step S21,
followed by terminating the present program.
If the answer to the question of the step S8 is affirmative (YES),
i.e if FXAVE=1, the timer tMW0TX is set to the predetermined time
period TMWOTX0 and started at a step S15, and it is determined at a
step S16 whether or not the second enriching flag FHSFE2 is equal
to 1. If the answer to this question is affirmative (YES), the
present program is immediately terminated, whereas if the answer is
negative (NO), i.e. if FHSFE2=0, it is determined at a step S17
whether or not the count value of the counter CHSFE set at the step
S9 is equal to 0. If the answer to this question is negative (NO),
i.e. if CHSFE>0, the count value is decreased by a decrement of
1 at a step S19, followed by the program proceeding to the step
S20. If the answer to the question of the step S17 is affirmative
(YES), i.e. if CHSFE=0, the second enriching flag FHSFE2 is set to
a value of 1 at a step S18, followed by terminating the present
program.
The setting of the first and second enriching flags FHSFE1, FHSFE2
according to the above steps S8 to S21 can be summarized as
follows:
(i) If FXAVE=0 and FWOT2=0, FHSFE1 and FHSFE2=0.
(ii) If FXAVE=0, and the predetermined time period TMWOTX0 has
elapsed after the second high load flag was changed from 0 to 1,
the first enriching flag FHSFE1 alone is set to 1.
(iii) If FXAVE is changed from 0 to 1, the first enriching flag
FHSFE1 is immediately set to 1 (in the case where it has already
been set to 1, it is held thereat), and when a time period
corresponding to the predetermined count value CHSFE0 has elapsed,
the second enriching flag FHSFE2 is set to 1.
FIGS. 14a and 14b show a program for calculating a high load
enriching coefficient KWOT applied to the aforementioned equation
(1), for enriching the supply air-fuel ratio when the engine is in
a high load condition. This program is executed whenever a TDC
signal pulse is generated, and in synchronism therewith.
At a step S101, a KWOT map in which values of the high load
enriching coefficient KWOT are set according to the engine
rotational speed NE and the intake pipe absolute pressure PBA is
searched to calculate a value of the high load enriching
coefficient KWOT (this value retrieved from the map is designated
as KWOTM). Then, it is determined at a step S102 whether or not the
second high load flag FWOT2 is equal to 1. If the answer to this
question is negative (NO), i.e. if FWOT2=0, a third high load flag
FWOT is set to a value of 0 at a step S114, and the high load
enriching flag KWOT is set to a value of 1.0 (correction value) at
a step S116 in FIG. 14b.
If the answer to the question of the step S102 is affirmative
(YES), i.e. if FWOT2=1, it is determined at a step S103 whether or
not the first high load flag FWOT1 is equal to 1. If the answer to
this question is negative (NO), i.e. if FWOT1=0, it is determined
at a step S104 whether or not the first enriching flag FHSFE1 is
equal to 1. If the answer to this question is negative (NO), i.e.
if FHSFE1=0, the program proceeds to the step S114, whereas if the
answer is affirmative (YES), i.e. if FHSFE1=1, it is determined at
a step S107 whether or not the second enriching flag FHSFE2 is
equal to 1. If the answer to this question is negative (NO), i.e.
if FHSFE2=0, the value KWOTM retrieved from the map at the step
S101 is set to the high load enriching coefficient KWOT without any
change at a step S110, followed by the program proceeding to the
step S113.
If the answer to the question of the step S107 is affirmative
(YES), i.e. if FHSFE2=1, a value of an enriching coefficient XWOTR
(>1.0) is read at a step S108 from an XWOTR table in which
values of the enriching coefficient XWOTR are set according to the
engine rotational speed NE, and a value obtained by multiplying the
value KWOTM retrieved from the map at the step S101 by the
enriching coefficient XWOTR is set as the high load enriching
coefficient KWOT at a step S109, followed by the program proceeding
to the step S113.
If the answer to the question of the step S103 is affirmative
(YES), i.e. if FWOT1=1, it is determined at a step S105 whether or
not the first enriching flag FHSFE1 is equal to 1. If the answer to
this question is negative (NO), it is further determined at a step
S106 whether or not the engine coolant temperature TW is higher
than a predetermined value TWHS (e.g. 95.degree. C.). If either the
answer to the question of the step S105 or the answer to that of
the step S106 is affirmative (YES), i.e. if FHSFE1=1 or TW>TWHS,
the program proceeds to the step S107.
If both the answers to the questions of the steps S105 and S106 are
negative (NO), i.e. if FHSFE1=0 and TW<TWHS, a value of a
leaning coefficient XWOTL (<1.0) is read at a step S111 from an
XWOTL table shown in FIG. 16, in which values of the leaning
coefficient XWOTL are set according to the engine rotational speed
NE similarly to the XWOTR table, and a value obtained by
multiplying the value KWOTM retrieved from the map at the step S101
by the leaning coefficient KWOT is set as the high load enriching
coefficient KWOT at a step S112, and then the program proceeds to
the step S113.
According to the above steps S101 to S112, the enriching of the
supply air-fuel ratio depending on the states of the first and
second enriching flags FHSFE1 and FHSFE2 can be summarized as
follows:
(i) If FHSFE1=1 and FHSFE2=0, the high load enriching coefficient
KWOT is set to the value KWOTM retrieved from the map (at the step
S110) to thereby control the air-fuel ratio to a value of
A/F=11.5.
(ii) If FHSFE2=1, the high load enriching coefficient KWOT is set
to the value obtained by multiplying the value KWOTM by the
enriching coefficient XWOTR (at the step S109) to thereby control
the air-fuel ratio to a value of A/F=10.0.
(iii) If FWOT1=FWOT2=1 and FHSFE1=0, the high load enriching
coefficient KWOT is set to the value obtained by multiplying the
value KWOTM by the leaning coefficient XWOTL to thereby control the
air-fuel ratio to a value of A/F=13.0.
As a result, the enriching of the supply air-fuel ratio can be
properly performed depending on the states of the enriching flags
FHSFE1 and FHSFE2, i.e. depending on the estimated temperature
values of the component parts of the engine (i.e. the state of the
high temperature flag FXAVE) determined by the program shown in
FIG. 2 and operating conditions of the engine (i.e. the states of
the high load flags FWOT1 and FWOT2), whereby fuel consumption and
emission of CO can be reduced.
At the step S113, the third high load flag FWOT is set to a value
of 1, and the program proceeds to a step S115 in FIG. 14b, where it
is determined whether or not the high load enriching coefficient
KWOT is larger than the engine coolant temperature-dependent fuel
increasing coefficient KTW. If the answer to this question is
negative (NO), i.e. KWOT.ltoreq.KTW, the program proceeds to the
step S116, whereas if the answer is affirmative (YES), i.e. if
KWOT>KTW, the engine coolant temperature-dependent fuel
increasing coefficient KTW is set to a value of 1.0 at a step S117,
and then a value obtained by multiplying the high load enriching
coefficient calculated at the step S109 or S110 or S112 by an
engine coolant temperature-dependent enriching coefficient XWOTTW
is newly set as KWOT at a step S118.
The engine coolant temperature-dependent enriching coefficient
XWOTTW is read from a table, shown in FIG. 17, in which values
XWOTTW0 to XWOTTW3 (e.g. 1.0, 1.05, 1.1O, and 1.15, respectively)
are set corresponding to predetermined values TWWOT0 to TWWOT3
(e.g. 90.degree. C., 100.degree. C., 111.degree. C., and
119.degree. C.) of the engine coolant temperature.
At a step S119, it is determined whether or not the high load
enriching coefficient KWOT calculated at the step S118 is larger
than a predetermined upper limit value KWOTX (e.g. 1.38). If the
answer to this question is negative (NO), the program immediately
proceeds to a step S121, whereas if the answer is affirmative
(YES), the high load enriching coefficient KWOT is set to the
predetermined upper limit value KWOTX, and then the program
proceeds to the step S121. At the step S121, it is determined
whether or not the high load enriching coefficient KWOT is larger
than a predetermined lower limit value KWOTE (1.31). If the answer
to this question is affirmative, the program is immediately
terminated, whereas if the answer is negative (NO), the high load
enriching coefficient KWOT is set to the predetermined lower limit
value KWOTE at at a step S122, followed by terminating the present
program.
According to the steps S119 to S122, when the high load enriching
coefficient assumes a value outside the range determined by the
predetermined upper and lower limit values, it is set to the
predetermined upper limit value KWOT or the predetermined lower
limit value KWOTE.
* * * * *